1. Field of Invention
This application relates to processes and systems for making particles, and more particularly processes and systems for making particles having a dimension less than about 1 mm and particles and compositions containing the particles.
2. Discussion of Related Art
The contents of all references, including articles, published patent applications and patents referred to anywhere in this specification are hereby incorporated by reference.
An important emerging class of non-spherical colloidal materials are microscopic and nanoscopic particles that have designed shapes and are created by lithographic means (see e.g. Hernandez, C. J.; Mason, T. G. Colloidal alphabet soup: Monodisperse dispersions of shape-designed LithoParticles. J. Phys. Chem. C 2007, 111, 4477-4480). (Shape-designed particles, regardless of the methods of production, will also be referred to as LithoParticles in this specification.) Optical pattern replicating systems, such as high-fidelity lens-based steppers (Madou, M. J. Fundamentals of microfabrication: The science of miniaturization. 2nd ed.; CRC Press: Boca Raton, 2002), typically used to print electronic structures on computer chips, have been used to mass-produce LithoParticles and create Brownian dispersions of an entire particulate alphabet: “Colloidal Alphabet Soup” (Hernandez, C. J.; Mason, T. G. Colloidal alphabet soup: Monodisperse dispersions of shape-designed LithoParticles. J. Phys. Chem. C 2007, 111, 4477-4480). In the basic implementation of this approach, a polymer resist layer can be cross-linked by the optical exposure and, after development, the polymer resist particles can be lifted off of the substrate (see U.S. application Ser. No. 12/377,773 filed Feb. 17, 2009 as a national stage application of PCT/US07/18365, entitled “Customized Lithographic Particles,” by the same assignee as the current application, the entire contents of which are hereby incorporated by reference). This optical approach for making LithoParticles has important and non-obvious differences from earlier approaches (Higurashi, E.; Ukita, H.; Tanaka, H.; Ohguchi, O. Optically induced rotation of anisotropic micro-objects fabricated by surface micromachining. Appl. Phys. Lett. 1994, 64, 2209-2210; Brown, A. B. D.; Smith, C. G.; Rennie, A. R. Fabricating colloidal particles with photolithography and their interactions at an air-water interface. Phys. Rev. E 2000, 62, 951-960; Sullivan, M.; Zhao, K.; Harrison, C.; Austin, R. H.; Megens, M.; Hollingsworth, A.; Russel, W. B.; Cheng, Z.; Mason, T. G.; Chaikin, P. M. Control of colloids with gravity, temperature gradients, and electric fields. J. Phys. Condens. Matter 2003, 15, S11-S18) that required destructive etching as part of the procedure.
Mechanical imprinting, whether thermal or step-and-flash, is a technology that involves bringing two solid surfaces into contact after depositing a desired material between them (Madou, M. J. Fundamentals of microfabrication: The science of miniaturization. 2nd ed.; CRC Press: Boca Raton, 2002; Chou, S. Y. Nanoimprint lithography and lithographically induced self assembly. MRS Bulletin 2001, 26, 512; Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Nanoimprint lithography. J. Vacuum Sci. Tech. B 1996, 14 (6), 4129-4133; Resnick, D. J.; Mancini, D.; Dauksher, W. J.; Nordquist, K.; Bailey, T. C.; Johnson, S.; Sreenivasan, S. V.; Ekerdt, J. G.; Willson, C. G. Improved step and flash imprint lithography templates for nanofabrication. Microelectronic Engineering 2003, 69, 412-419). Once the surfaces of the two plates touch, the material only fills trenches or wells in one plate that has been prepared with the desired relief patterns in the surfaces of the plates. Imprinting essentially forces a desired material into voids that have been created in one of the surfaces to form a mold. While the two plates are touching (or nearly touching), a process, such as cross-linking in the case of polymers, can be used to rigidify the material in the mold, and then the plates are separated. During the separation, if the release of the desired material from the corrugated surface can be made efficiently, then the result is a set of raised structures of the desired material on the flat surface of the other plate. Imprinting is a subset of the more general process of embossing, in which a mold is pressed into the surface of a material that is not as rigid and then removed to create raised corrugations that reflect the mold. However, by contrast to embossing, mechanical imprinting involves squeezing out material between two solid plates where they touch, so that only the negative relief corrugations in one plate become filled with the desired material.
Performing mechanical imprinting reproducibly in a production setting can be problematic for many reasons. It is often difficult to achieve good mechanical contact between the two plates over large surface areas. To mitigate this, large sections of the plates are often cut away so that only small, disconnected pedestals containing the desired patterns touch the flat plate. Using pedestals decreases the surface area and production rate significantly. Defects in the surfaces of the plates, dust, or enhanced surface roughness due to wear can preclude the exact contact of the plates, especially for larger substrate sizes. For very small shapes, the wetting properties of the material to be imprinted with the plates can play an important role in determining the success and reproducibility of the imprinting procedure. These are some of the primary reasons why mechanical imprinting has not been widely adopted by the electronics industry as a replacement to more reliable optical approaches. Although it is possible to create LithoParticles using imprinting methods, as we and others have demonstrated (see e.g. Rolland, J. P.; Maynor, B. W.; Euliss, L. E.; Exner, A. E.; Denison, G. M.; DeSimone, J. M. Direct fabrication of monodisperse shape-specific nanobiomaterials through imprinting (J. Am. Chem. Soc. 2005, 127, 10096-10100)), developing alternative approaches for rapidly mass-producing LithoParticles that do not involve mechanical imprinting would be highly useful.
One method of producing particles that does not involve mechanical imprinting is relief deposition templating. In this method, a patterned relief surface is created on a solid substrate, and a deposition of a particle material is made in a manner that creates discrete regions that can be separated from the template and retain a geometrical feature imparted by the template. Two implementations of this are pillar deposition templating (Hernandez, C. J.; Zhao, K.; Mason, T. G. Pillar-deposition particle templating: A high-throughput synthetic route for producing LithoParticles (Soft Materials 2007, 5, 1-11)) in which the particles are formed on the top surfaces of pillars (i.e. relief projections), and well deposition templating (Hernandez, C. J.; Zhao, K.; Mason, T. G. Well-deposition particle templating: Rapid mass-production of LithoParticles without mechanical imprinting (Soft Materials 2007, 5, 13-31)), in which the particles are formed by wells (i.e. relief depressions) in the template. The relief deposition templating method offers several advantages over mechanical imprinting, but it typically cannot be used if the deposited material forms an interconnected region over the relief structures that preclude facile separation of the particles from the template. Thus, it would be highly useful to further structure and pattern material deposited onto a patterned relief structure, including an interconnected layer of deposited material, on the template.
In addition, to more rapidly produce particles having more advanced complex three-dimensional shapes, it would be highly useful to combine the existing methods of radiation deposition templating (e.g., U.S. application Ser. No. 12/563,907 assigned to the same assignee as the current application which is entitled “Mechanical Process for Creating Particles in a Fluid” filed on Sep. 21, 2009 as a CIP of PCT/US08/03679, the entire contents of which are incorporated herein by reference), which overcomes limitations of imprinting methods and which does not involve exposure to spatially patterned radiation (U.S. application Ser. No. 12/377,773), with the existing methods of spatially patterned radiation. The shape of the particle would then be designed by a combination of patterned surface relief and spatially patterned radiation. This combination would provide a versatility and efficiency for making shape-designed particles that can be superior to either of these methods on their own. Consequently, there remains a need for improved processes and systems for making particles having a dimension less than about 1 mm.